In a typical cellular telephone system, as a mobile unit travels along a path that passes from one cell to another, a handoff occurs. The handoff action is controlled by a mobile telephone switching office (MTSO) which receives a handoff command or instruction. The handoff command is typically generated when the signal received from the mobile telephone falls below a preselected signal strength thus indicating that the mobile telephone is at the cell boundary.
Each cell in a cellular telephone system operates with a different assigned set of transmission frequencies. As a mobile telephone passes from one cell to another, the handoff signal instructs the cell which the mobile telephone is entering to begin transmitting at a frequency which is different from the frequency which was being transmitted by the cell which the mobile telephone is leaving. A similar procedure is followed when the mobile telephone passes into the next contiguous cell. Sets of assigned frequencies are different for adjacent cells, and such sets are not repeated except for cells that are far enough away from each other so that interference problems will not occur.
A mobile telephone unit typically contains a control unit, a transceiver, and an antenna system. Each cell site typically is provided with a control unit, radio cabinets, a power plant, data terminals, and antennas. The MTSO provides coordination for all the cell sites and contains suitable processing and switching means. The MTSO also interfaces with the telephone company zone offices for standard hardwired telephone systems. The communication links between the MTSO and the various cell sites are typically microwave, T carriers, or optical fiber, and carry both voice and control data between the cell sites and the MTSO.
When a user sitting in a car activates the receiver of the mobile unit, the receiver scans a plurality of set-up channels which are designated among the total channels assigned to the cell. Typically, there may be 21 set-up channels out of a total of 416 channels. (The remainder are communication channels.) The receiver then selects the strongest set-up channel and locks on for a certain time. Each site is assigned a different set-up channel. Accordingly, locking onto the strongest set-up channel usually means selecting the nearest cell site. This self-location scheme is used in the idle stage and is user-independent. It has a great advantage because it eliminates the load on the transmission at the cell site for locating the mobile unit. The disadvantage of the self-location scheme is that no location information of idle mobile units appears at each cell site. Therefore, when the call initiates from a standard non-mobile or land line to a mobile unit, the paging process is longer. Since a large percentage of calls originate at the mobile unit, the use of self-location schemes is justified. After a delay (for example, one minute) the self-location procedure is repeated.
To make a call from a mobile unit, the user places the called number into an originating register in the mobile unit, checks to see that the number is correct, and pushes a "send" button. A request for service is sent on a selected set-up channel obtained from a self-location scheme as described above. The cell site receives it, and in directional cell sites, selects the best directive antenna for the voice channel to use. At the same time the cell site sends a request to the MTSO via a high-speed data link. The MTSO selects an appropriate voice channel for the call, and the cell site acts on it through the best directive antenna to link the mobile unit. The MTSO also connects the wire-line party through the telephone company zone office.
When a land-line party dials a mobile unit number, the telephone company local office recognizes that the called number is mobile and forwards the call to the MTSO. The MTSO sends a paging message to certain cell sites based on the mobile unit number and a suitable search algorithm. Each cell site transmits the page on its own set-up channel. The mobile unit recognizes its own identification on a strong set-up channel, locks onto it, and responds to the cell site. The mobile unit also follows the instruction to tune to an assigned voice channel and initiate user alert.
When the mobile user turns off the transmitter, a particular signal (signaling tone) transmits to the cell site and both sides free the voice channel. The mobile unit resumes monitoring pages through the strongest set-up channel.
During a call, two parties are on a voice channel. When the mobile unit moves out of the coverage area of a particular cell site, the reception becomes weak. The present cell site requests a handoff via an appropriate signal, for example, a 100 ms burst on the voice channel. The system switches the call to a new frequency channel in a new cell site without either interrupting the call or alerting the user. The call continues as long as the user is talking. The user does not notice the handoff occurrences.
When call traffic in a particular area increases, increased capacity may be generated by reducing the area covered by a particular cell. For example, if a cell is split into four smaller cells, each with a radius of one-half the original, traffic is increased four fold. Naturally, the smaller the cell, the more handoffs required in a cellular telephone system for a given capacity.
Although in the proper circumstances reduced cell size is advantageous, certain problems can arise. Very often when cell size is reduced, for example to a radius of less than one mile, very irregular signal strength coverage will result. This may be caused by buildings and other structures, and can therefore become highly dependent upon the location of the mobile unit. Other problems arise in connection with signal interference. Although some cellular telephone systems, in an attempt to improve coverage, have employed several sets of frequencies in a small single cell, this prevents the reuse of the same frequencies or adjacent frequencies in the neighboring cells. The overall capacity of the system thereby decreases, since the number of available channels in a system is proportional to the inverse of the number of different frequency sets employed.